The seasonal and intraseasonal variability of diurnal cloud activity over the Tibetan Plateau
The seasonal and intraseasonal variability of diurnal cloud activity over the Tibetan Plateau
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DOI：
10.2151/jmsj.79.1207
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发表时间：
2001-12-01
影响因子：
3.1
通讯作者：
Yasunari, T
中科院分区：
文献类型：
--
作者：
Fujinami, H;Yasunari, T
利用1989-1994年6年的静止气象卫星（GMS）3小时资料，分析了青藏高原全年云日活动（DCA）的季节变化。DCA在季节性周期中显示出两个不同的方差最大值。一个是在春季（前季风季节），另一个是在夏季季风季节。DCA从1月下旬开始，从3月到4月达到最大值。DCA活动范围几乎遍及整个高原，尤其是高原南部（30 ° N，90 ° E）和35 ° N，80 ° E和31 ° N，102 ° E之间的带状地带。两个活跃的DCA之间的一个短的时间间隔被发现在5月下旬至6月初。DCA于6月再次从高原东南部地区（中心位于北纬30度，东经101度）开始，并向南移动。从7月到8月，DCA在南部地区（大约30度N，86度E）最为活跃。9月以后，活跃的DCA向东南部地区撤退，春季和夏季DCA的振幅都在季节内时间尺度上波动。春季DCA活跃期，对应于高空中纬度西风带槽向高原南部弯曲，高原上空高层冷空气团明显，高原对流层风速弱。这种大气结构和白天低层大气的加热可能是春季DCA增强的原因。而在夏季风季节，高原低层大气湿度和温度的增加，与南亚季风区的暖湿空气侵入高原低层大气和对流活动引起的降水有关。这表明湿对流的不稳定性增加。这些大气环流特征和地面加热在白天引起活跃的湿对流。与高原南部DCA增强相对应，青藏高压中心位于高原南部。本文还讨论了DCA季节内变化与高层大气环流季节内变化相联系的可能机制。
Seasonal variation of diurnal cloud activity (abbreviated DCA) over the Tibetan Plateau throughout the year is examined using 3-hourly geostationary meteorological satellite (GMS) data for 6-years (1989-1994). The DCA shows two distinct variance maxima in the seasonal cycle. One is in spring (pre-monsoon season), and the other is in the summer monsoon season. The DCA begins in late January, and reaches its maximum from March through April. The active DCA extends over almost the whole of the plateau, especially over the southern part (around 30degreesN, 90degreesE) and the zonally oriented belt between 35degreesN, 80degreesE and 31degreesN, 102degreesE. A short interval between the two active DCAs is found around late May to early June. The DCA starts again over the southeastern region of the plateau (centered around 30degreesN, 101degreesE) in June and moves up to the southern region. From July to August, the DCA is most active over the southern region (around 30degreesN, 86degreesE). After September, the active DCA retreats to the southeastern region.In both spring and summer, the amplitude of the DCA fluctuates on intra-seasonal time scales. In the active period of the DCA in spring, corresponding to the meandering of the upper-level mid-latitude westerly to the south of the plateau with a trough, the cold air mass at the upper-level is clearly seen over the plateau and weak wind speed is observed through the plateau troposphere. This atmospheric structure and a heating of the lower atmosphere during daytime are likely to be responsible for the enhancement of the DCA in spring. In contrast, during summer monsoon season, the increase of humidity and temperature are identified at the lower atmosphere over the plateau, associated with a humid and warm air intrusion from the South Asian monsoon area into the lower atmosphere over the plateau and precipitation due to active convection. This indicates an increase of the instability for moist convection. These features of the atmospheric circulation and the surface heating during daytime induce active moist convection. Corresponding to the enhanced DCA over the southern part of the plateau, the center of the Tibetan high is located over there. A possible mechanism for the intraseasonal variability of the DCA associated with that of the upper-level atmospheric circulation is also discussed.